Eating more nuts, fruits, vegetables, whole grains and yogurt can keep help keep age-related weight gains in check. In what could almost pass for deadpan humor, the authors write, "Obviously, such foods provide calories and cannot violate thermodynamic laws."

Potatoes — chips, fries and plain — lead the list of foods that can pack on the pounds.

So what's going on? They figure that eating more of these foods may have led to lower consumption of foods that are higher in calories.

And how could that be? More fiber and slower digestion of these foods could be part of the answer.

Yogurt, though, was also a winner, and it's no fiber bomb. The researchers aren't exactly sure how it helps keep waistlines in check, though it could have something to do with good work the bacterial cultures do inside our intestines. It's also possible that people who eat a lot of yogurt have other healthful habits.

The worst of the bad food news can be summed up in one word: potatoes.

Potato chips, french fries, mashed potatoes, boiled potatoes and baked potatoes were associated with the biggest weight gains over time. For every additional daily serving of potatoes people ate, they gained more than 1 1/4 pounds over a four-year period.

After taters, sugary drinks (sodas!), unprocessed red meat, and processed meats, such as hot dogs and bacon, were the worst offenders.

Information about what the people ate, and how much of it, came from questionnaires rather than direct measurements.

The bottom line: little changes in diet could make a big difference in weight over time. Mozaffarian told the Wall Street Journal the study's findings suggest "that the path to eating fewer calories is not simply to count calories, but to focus on consuming a more healthy diet in general."